Top panel: ratio [O i] 557.79/[O i] 630.03 as function of L_acc for stars in Lupus (red dots) and σ Ori (filled blue squares). Arrows indicate 3σ upper limits. Objects where both a LVC and a HVC are detected are marked by large open circle. Black crosses show the values for the set of DR Tau spectra obtained at different epochs, and green open squares are stars in Taurus (see text). Bottom panel: model predictions for collisions with electron (solid curves) and with hydrogen atoms (dashed curves). Different curves refer to different temperatures in units of 10⁴ K, as labeled. The n_e scale is shown in the bottom axis, that for n_H on the top axis. The gray area shows the location of the observed values, marked by the two dashed lines in the top panel.